1. Field of the Invention
The present invention generally relates to thermographic imaging and more particularly relates to pulsed thermography.
2. Description of the Related Art
Active thermography is used to nondestructively evaluate (NDE) samples for sub-surface defects. It is effective for uncovering internal bond discontinuities, delaminations, voids, inclusions, and other structural defects that are not detectable by visual inspection of the sample. Generally, active thermography involves heating or cooling the sample to create a difference between the sample temperature and the ambient temperature and then observing the infrared thermal signature that emanates from the sample as its temperature returns to ambient temperature. In application, pulsed thermography NDE is widely used in evaluating, for example, aerospace and power generation industry devices.
An infrared (IR) camera is typically used for thermography because it is capable of detecting anomalies in the cooling behavior of the sample are commonly caused by sub-surface defects blocking the diffusion of heat from the surface of the sample surface to the sample's interior. More particularly, subsurface defects cause the surface immediately above the defect to cool at a different rate than that of the surrounding defect-free areas. As the sample cools, the IR camera captures and records an image of the sample, creating a sequential time record of the sample's surface temperature.
Systems for thermographic heating typically employ xenon flashtubes and off-the-shelf photographic power supplies for sample excitation. In an alternative embodiment, a laser may be employed to heat a surface; however, lasers are less practical when used to instantaneously heat a large area. The use of commercial off-the-shelf power supplies is extremely convenient, cost effective, and safe.
Off-the-shelf flashlamps (and their associated power supplies which provide current pulses with energies on the order of 3–6 kJoules) are used to generate a plasma arc in a quartz flashtube filled with xenon gas with full-width-half-maximum (FWHM) durations ranging from 2–5 milliseconds. It is the light output from this plasma arc that serves as the heat source in pulsed thermography NDE. Commercial power supply/flashlamp combinations from manufacturers such as Speedtron and BALCAR®, which were originally designed for professional photographers, are now used almost exclusively by practitioners of pulsed thermography NDE.
In configuring the flashlamps for pulsed thermography NDE, quasi-parabolic reflectors in symmetric pairs arranged about the IR camera typically house the flash lamps. As such, the flash lamps are generally placed a few feet away from the sample, so that a small sample positioned at the intersection of two flashlamp beam paths will be illuminated in an approximately uniform manner. This arrangement provides satisfactory uniformity as long as there is sufficient space to allow the lamps to be adequately far from the sample. However, since the intensity of light reaching the sample decreases as the inverse square of the distance between the lamp and the sample, practitioners often use several pairs of high-energy flashlamps and power supplies to compensate for the lost energy.
Several drawbacks result from the standard practice of using photographic equipment in pulsed thermography NDE. For example, the direct energy projected onto the sample by the plasma arc is much greater than the indirect energy projected by the reflector, so that the actual spatial temperature distribution at the sample surface may be quite non-uniform, with “hot spots” in the areas where the direct energy of the plasma arc was projected onto the sample. Invariably, in describing pulsed thermography, the use of a “brief, spatially uniform pulse of light” is mentioned; however, close scrutiny of the current practice employed in pulsed thermography NDE clearly shows that the generated light pulses are neither spatially uniform, nor brief, when considered on the scale of thermal diffusion times.
Although cameras operating at the standard (50/60 Hz) frame rate are adequate in most NDE applications, high speed cameras may be used in some particular NDE applications. However, high speed cameras are significantly more expensive and generally require some trade-off in pixel density (i.e. the pixel count must be reduced as the frame rate is increased, in order to maintain operation within a fixed bandwidth limit). Furthermore, IR cameras can be rendered ineffective by conventional thermal excitation schemes that are widely used because these schemes can cause causes detector saturation or nonlinear camera response (especially in the frames proximate to the excitation by the light pulse). As a result, true high speed, high resolution pulsed thermography NDE is rarely performed, and can only be done with extremely specialized (and expensive) equipment.
Even further, there are several negative consequences associated with the conventional pulsed thermography NDE, some of which are:
1. The equipment is large and cumbersome, and does not lend itself to applications where a small portable unit is to be deployed, such as aircraft inspection.
2. Although the FWHM of the flash pulse is in the 2–5 milliseconds range, there is also a substantial IR component to the flash that causes a significant afterglow effect that is typically 25–35 milliseconds in duration. Thus, the afterglow tends to cause the IR camera to saturate or yield a non-linear response.
3. The energy from the afterglow may mask weaker signals emitted from the sample that indicate features or defects residing near the surface of the sample under examination.
4. Although high-speed cameras at frame rates up to 1.5 kHz are commercially available, the performance of these cameras is not well integrated with to the characteristics of the off-the-shelf flashlamps described above. The duration of the flash and the subsequent tail tends to render the camera “blind” for several frames, so that data that corresponds to “near surface features” is lost.
5. Analysis of pulsed thermography data is generally performed on the time history of the sample surface temperature response as it returns to room temperature. However, as the temperature approaches ambient, noise effects become more pronounced and the signal to noise ratio decreases. As a result, it becomes difficult to detect deeper defect features, as weaker signals are masked by noise from the camera electronics or stray reflected radiation.
6. The light intensity at the sample surface is not uniform unless the sample is placed in the far field of the lamps.
Thus, a need exists for an improved pulsed thermography system that overcomes the deficiencies of off-the-shelf photographic products that were originally designed for commercial photographers.